mu4e: split off mime-handling from mu4e-view & improve

Split off the MIME-handling from mu4e-view.el into its own
mu4e-mime-parts.el

Improve the implementation, updating completions with annotations.
Support "Pick all" for completing attachments and MIME-types.

Both attachment/inline disposition MIME-type with a file name are
considered "attachment" now.

Allow MIME-part actions to target multiple MIME-parts.

mu4e/meson.build

@@ -45,6 +45,7 @@ mu4e_srcs=[
   'mu4e-main.el',
   'mu4e-mark.el',
   'mu4e-message.el',
+  'mu4e-mime-parts.el',
   'mu4e-modeline.el',
   'mu4e-notification.el',
   'mu4e-obsolete.el',